The building or decorating of homes requires a wide variety of fastening operations in a wide variety of contexts. For example, various moldings and wall coverings need to be fastened onto wall surfaces, such as plaster, wall board or gypsum board. There are a variety of ways currently known to fasten surfaces together which are conventionally used in the building trades. Examples of such commonly used fasteners include nails, screws, staples, and adhesives or glues. Each of these methods has its own set of advantages and drawbacks. For example, while nails, screws and staples are relatively easy to use, they tend to present difficulties in a decorative context since they can mar the surfaces being fastened and detract significantly from the decorative look of the surface being applied. Glues or other adhesives can be messy, difficult to apply to surfaces and tend not to be reversible, as anyone who has ever tried to strip wallpaper from a wall can readily attest. It would be highly desirable, therefore, to have a fastener which is not only strong and easy to use, but which does not significantly mar either of the surfaces being fastened and which, if desired, can be readily reversed allowing the fastened surfaces to be easily separated. That is what the present invention accomplishes.
The use of electromagnetic energy (i.e., induction heating) to activate adhesives is known in the art. However, the art does not teach or suggest a fastening device, activated by electromagnetic energy, which forms a strong bond which may be easily reversed, such as that described in the present application. In fact, most technical development in this area has centered on permanent, high strength adhesive bonding of aerospace or automotive assemblies where very high bond strengths, e.g., greater than 4,000 psi, are required. Bond times are long to accommodate the slow, repeatable, uniform development of bond strengths over long cure times, for example from one minute to several hours to upwards of a day. In contrast, for many construction and other manufacturing applications, the bond strengths do not have to exceed 2,500 psi, or in many cases, 1,000 psi. Furthermore, the bond strengths do not have to be stringently uniform or as tightly repeatable. Finally, and importantly, it is highly desirable that the bond times be very fast, less than one minute, usually less than ten seconds, and frequently even less than two seconds. This is also what the present invention accomplishes.
While microwave heating with hysterectically activateable magnetic particles is a solution in many respects to the above-described problem, microwave heating may be unsafe for use in a human environment and must be used in a highly controlled environment to minimize these safety concerns. The present invention permits such fastening to be carried out both in a controlled manufacturing plant environment and in a much less controlled construction site environment.
The use of thermoplastic welds to join metal surfaces, for example in the aircraft industry, is disclosed in a number of patents. Some of these patents utilize structural susceptors which provide a vehicle for carrying the thermoplastic adhesive resin and for reinforcing the bond formed. The key in these patents is to maximize the contact between the surfaces to be bonded and the adhesive layer thereby forming a bond which is exceptionally strong and not reversible. Another focus of these patents is the elimination of the edge effect when the weld is exposed to electromagnetic energy, in order to obtain uniform heating across the weld thereby forming a bond of uniform strength. See, for example, U.S. Pat. No. 5,717,191, Christensen, et al., issued Feb. 10, 1998; U.S. Pat. No. 5,500,511, Hansen, et al., issued Mar. 19, 1996; U.S. Pat. No. 5,508,496, Hansen, et al., issued Apr. 16, 1996; U.S. Pat. No. 5,705,795, Anderson, et al., issued Jan. 6, 1998; U.S. Pat. No. 5,705,796, Hansen, et al., issued Jan. 6, 1998; U.S. Pat. No. 5,723,849, Matsen, et al., issued Mar. 3, 1998; U.S. Pat. No. 5,756,973, Kirkwood, et al., issued May 26, 1998; U.S. Pat. No. 5,847,375, Matsen, et al., issued Dec. 8, 1998; U.S. Pat. No. 5,916,469, Scoles, et al., issued Jun. 29, 1999; U.S. Pat. No. 5,919,387, Buckley, et al., issued Jul. 6, 1999; and U.S. Pat. No. 5,919,413, Avila, issued Jul. 6, 1999.
Portable hand-held induction tools used for forming thermoplastic welds based on induction heating are also known. See, for example, U.S. Pat. No. 4,017,701, Mittelmann, issued Apr. 12, 1977; U.S. Pat. No. 5,266,764, Fox, et al., issued Nov. 30, 1993; U.S. Pat. No. 5,350,902, Fox, et al., issued Sep. 27, 1994; U.S. Pat. No. 5,374,808, Coultrip, et al., issued Dec. 20, 1994; U.S. Pat. No. 5,374,809, Fox, et al., issued Dec. 20, 1994; U.S. Pat. No. 5,874,713, Cydzik, et al., issued Feb. 23, 1999; and U.S. Pat. No. 5,378,879, Monovoukas, issued Jan. 3, 1995.
Known induction heating devices generally suffer from an inability to be made truly portable, i.e., lightweight, while simultaneously delivering the level of energy necessary to form bonds in short periods of time. Part of the reason that such portable induction heating devices do not exist is that there are no fastening devices which can bond quickly and effectively upon the application of low levels of power. It would, therefore, be very useful to have an adhesive device which utilizes such low levels of energy very efficiently, for example, by accumulating heat in the susceptor and adhesive while simultaneously minimizing conduction losses to the substrate (surface to be bonded) until the adhesive has melted, begun chemical reaction, flowed adequately, or all three. Such a device to be useful should provide bonding strong enough to meet a wide variety of applications. Finally, it would be beneficial if the bonds formed could be reversible. The fastening devices of the present invention can accomplish these objectives.
U.S. Pat. No. 4,038,120, Russell, issued Jul. 26, 1977, describes the use of an energized heating element or wire to heat a hot melt glue resulting in adhesion between contiguously assembled panels. The disclosed method involves heating a glue-coated wire to liquefy the glue, producing a cohesive state and facilitating the assembly of panels. This method is said to be useful for introducing a cohesive material (glue) to an area of limited accessibility (groove). The use of the heating element (wire) requires the direct application of energy (electricity) to provide the heat to melt the glue.
U.S. Pat. No. 3,996,402, Sindt, issued Dec. 7, 1976, relates to the assembly of sheet materials by the use of a fastening device utilizing an apertured sheet of any current-conducting material sandwiched between coatings of hot melt glue. An induction heating system is activated causing eddy current heating in the current-conducting material with consequent melting of the hot melt glue, thus resulting in fusion and bonding of the sheet materials.
U.S. Pat. No. 3,574,031, Heller, et al., issued May 26, 1970, describes a method and material for welding thermoplastic bodies by using a susceptor sealant between the bodies to be joined. The susceptor sealant is characterized by having particles, heatable by induction, dielectric or radiant energy, dispersed in a thermoplastic carrier compatible with the thermoplastic sheets to be welded. The welding of the thermoplastic sheets takes place by exposing the susceptor sealant to heat energy, softening the carrier material and joining all thermoplastic materials.
U.S. Pat. No. 3,612,803, Klaas, issued Oct. 12, 1971, discloses a fastening device which consists of a quantity of heat-activateable adhesive containing a closed electrically-conductive loop and a ferromagnetic material insulated from said closed loop. In operation, the fastening device is activated by a solenoid coil energized with alternating electrical current. The current emitted from the solenoid is transferred to the fastening device where a current of large amperage and low voltage is generated in the loop enveloped by the heat-activateable adhesive. The current produces heat that causes the adhesive to become sticky. The efficiency of the disclosed device is improved by fitting it with a ferromagnetic core enclosed within the closed loop.
U.S. Pat. No. 5,770,296, Remerowski, et al., issued Jun. 23, 1998, describes a fastening device which is useful in facilitating the assembly of associated parts by employing a heat-activated assembly element, such as dowel or a disk or a strip constructed to include a target material, and a solid substance which will exhibit adhesive and expansive properties on exposure to heat. The heat is generated in the target material by exposing the target material to electromagnetic waves. See also, U.S. Pat. No. 5,935,369, Remerowski, et al., issued Aug. 10, 1999; U.S. Pat. No. 5,932,057, Remerowski, et al., issued Aug. 3, 1999; and U.S. Pat. No. 5,938,386, Remerowski, et al., issued Aug. 17, 1999.